1. Field
Disclosed herein is a method for the production of magnetic powder composite cores pressed from a mix of alloy powder and binder. Also disclosed herein is a magnet core produced from a mix of alloy powder and binder and to an inductive component with a magnet core of this type.
2. Description of Related Art
Magnet cores, which are for example used in switched power supplies as storage chokes or as choke cores on the system input side, have to have a low permeability which must not be changed significantly either by a varying AC modulation or by a constant magnetic field superimposed on the AC modulation. For such applications, ferrite cores with an air gap have proved useful for the currently preferred operating frequencies in the range of some ten to a hundred kHz, while magnetic powder composite cores are used for higher-rated equipment.
Depending on operating frequency, the required storage energy and available space, various alloys can be considered for the production of these metal powder composite cores. In the simplest case, pure iron powders are used, but if superior magnetic properties are required, FeAlSi-based crystalline alloys (SENDUST) or even NiFe-based alloys are preferred. The most recent developments favour the use of rapidly solidified amorphous or nanocrystalline iron-based alloys. Amorphous FeSiB-based alloys, in particular, appear to offer advantages compared to classical crystalline alloys owing to their high saturation inductance, their low particle thickness due to manufacturing methods, and their high resistivity. Apart from the alloy itself, other factors such as a high packing density of the powder composite core are also highly relevant if the magnet core is to have a high storage energy or a high DC pre-loadability.
U.S. Pat. No. 7,172,660 B2 discloses powder composite cores produced from a rapidly solidified amorphous iron-based alloy, wherein a particularly high packing density of the magnet core is obtained by using a powder with a bimodal particle size distribution. The use of rapidly solidified amorphous alloys rather than crystalline alloys poses the problem that pressing at moderate temperatures does not result in a viscous flow of the powder particles, so that higher packing densities are difficult to obtain.
According to U.S. Pat. No. 5,509,975 A, high packing densities can also be obtained by pressing the powder to form a magnet core at temperatures slightly below the crystallisation temperature of the alloy used. However, the magnet cores produced in this way have a relatively high relative permeability and are therefore not suitable for applications where a maximum storage energy is required.
In addition, the relative permeability of these magnet cores changes significantly, in particular in the range of low modulations with constant magnetic fields. This is due to the marked platelet shape of the powder particles produced by the comminution of rapidly solidified strip. As a result, the powder particles are in the pressing process oriented with their face normal in the pressing direction, and the starting permeability becomes extremely high, particular at a high packing density, followed by a marked reduction in relative permeability as constant magnetic field modulation increases. This effect is described analytically in F. Mazaleyrat et al.: “Permeability of soft magnetic composites from flakes of nano crystalline ribbon”, IEEE Transactions on Magnetics Vol. 38, 2002. This behaviour is undesirable in magnet cores used as storage chokes or as chokes for power factor correction (PFC chokes) in pulsed power supplies.